ECN Magazine: Taking Lithium-Ion Batteries To New Extremes

Just like Goldilocks and her proverbial porridge, lithium-ion batteries
(LIBs) perform best when the temperature range is just right—that is,
neither too hot nor too cold. But this is a huge limiting factor when it
comes to using LIBs in electric vehicles (EVs) in many locales where
temperatures vary widely. LIBs perform poorly in extremes of heat or
cold, and this is one roadblock preventing a transition to the wider use
of EVs. As the authors of the study to follow note, "out of the 51
metropolitan areas in the United States, 20 areas normally experience
extreme cold days below –18° C (0° F ) while the summertime temperatures
in 11 areas (including overlaps with the former 20) routinely exceed 38°
C (100° F)." Similar temperature variations certainly exist throughout
major urban areas worldwide, and likewise represent a barrier to the
uptake of EVs as a potential renewable energy transport solution.

In a recent paper published in Nature Energy, however, a group of UC
Berkeley researchers report a novel invention that promises to
effectively mitigate the effects of thermal extremes when used with
LIBs. Their paper, entitled "Efficient thermal management of Li-ion
batteries with a passive interfacial thermal regulator based on a shape
memory alloy," details the contemporary operational landscape of LIBs in
relation to ambient temperature variations in various locales, but also
with regard to other confounding factors, such as newer fast charging
and discharing batteries, which further complicate heat management
strategies. They note that traditional linear thermal components
typically fail to manage both extremes of hot and cold, and other
potential solutions, such as controlled fluid loops, do not provide a
high enough ON/OFF contrast, not to mention cost and weight
considerations when used with EVs. Their solution is "a fluid-free,
passive thermal regulator that stabilizes battery temperature in both
hot and cold extreme environments. Without any power supply or logic,
the thermal regulator switches its thermal conductance according to the
local battery temperature and delivers the desirable thermal
functionality, retaining heat when it is cold and facilitating cooling
when it is hot."

To achieve this effect, their passive thermal regulator design draws on
two key nonlinear features from existing thermal regulator concepts. The
first of these features, solid-state phase change, exhibits good
abruptness in response to temperature change, but fails to achieve an
adequately high switching ratio (SR)—that is, the ON/OFF state thermal
conductance ratio—which is the prime performance metric for thermal
regulators. The second feature, the opening and closing of a thermal
interface, has a much higher SR but relies on the differential thermal
expansion between two materials. When the interface gap between
materials is closed, it exhibits strong nonlinear thermal conductance.
However, because the thermal expansion effect is relatively weak here,
this design requires an unduly large thermal regulator body to
accomplish the opening and closing of the gap.

As complicated as the preceding examples may sound, their solution—which
embodies aspects of both solid-state phase change and interfacial
thermal contact conductance—is remarkably simple. To achieve their
design goals, the study authors rely on a shape memory alloy (SMA) made
from Nitinol, a flexible nickel /titatnium alloy wire which is routed
around the periphery of a top thermal regulator plate, on which sit the
LIBs. The ends of the SMA wire, one corresponding to each corner of the
thermal regulator, connect with a bottom heat-sinking plate, known as a
thermal interface material (TIM). The top and bottom plates are held in
opposition by a set of four bias springs, which create a 0.5 mm air gap
between the top and bottom plates and also hold the SMA wire in a state
of tension. This defines the thermal-insulative OFF state.

As the battery heats up, the SMA, due to a undergoing phase
transformation, begins to contract and pull the two plates closer.
Thermal conductance is very low until the two plates touch, at which
point the force of the contracting wire is greater than the opposing
force of the bias spring, and the TIM plate (bottom) contacts the
thermal regulator plate holding the batteries (top), and begins
dissipating heat; this situation defines the ON state. The prototypal
model described here encapuslates the essence of the passive interfacial
thermal regulator.

To validate the fundamentals of this concept with regard to the SMA wire
and the bias springs, the study authors built a model and tested it in a
vacuum chamber, using two thermocoupled stainless steel bars as a heat
source and a heat sink—these corresponding to the top and bottom plates
here, respectively. In the experiment, thermal isolation in the OFF
state proved to be excellent, as confirmed by the very large temperature
discontinuity at the interface and the small temperature gradients
measured in each of the stainless steel bars. However, when the upper
bar temperature exceeded the SMA transition temperature, the gap closed
and the TIM (the lower bar) began to heat up considerably. The authors
note that the switch process here ocurred rapidly, within about 10
seconds, and that a record SR was achieved at 2,070:1. They point out
that the Nitinol SMA wires had to first be pre-conditioned under higher
stress loads before they could be relied upon to produce a stable,
repeatable response through many cycles.

With the proof-of-concept established, the researchers moved on to
demonstrating the concept in practice with two Panasonic 18650PF LIBs
sandwiched between aluminum plates, tested in an environmental chamber.
The design here used a similar thermal regulator design modified to fit
the dimensions of the batteries in their holder, which callled for
longer SMA wire lengths and a gap of around 1 mm between the top and
bottom plates. Also, to meet a high level of performance, it was crucial
to insulate the parallel thermal pathways of the wires and the springs
and the LIBs themselves with an aerogel blanket. To compare performance,
the researchers also provided two standard linear models, "always OFF"
and "always ON," which involved replacing the SMA with stainless steel
wires configured for a constant gap or constant contact between the two
plates, respectively.

Under experimental conditions ranging from –20° C ( –4° F; very cold) to
45° C (114° F; very hot), the thermal regulator performed well, warming
quickly from –20° C ( –4° F) to around 20° C (68° F) due to heat from
the battery retained by the air gap and increasing the uable factor of
the battery by a factor of three. At the opposite extreme, the thermal
regulator also performed admirably, transitioning to the ON state at
around 45° C (113° F) whereafter the temperature rise in the LIBs was
limited to 5° C (9° F). After testing this thermal regulator set-up
through 1,000 ON/OFF cycles, the investigators found the OFF state
performance to be just slightly degraded (an 8.5% battery capacity
reduction at –20 ° C [–4° F]) whereas the ON state performance remained
unchanged.

As the study authors note, the costs of their thermal regulator are
minimal when used with the standard "always ON" thermal management
approach, which would already include a TIM heat sink. The additoinal
mass of the SMA and bias springs is less than a gram, and the cost of
the Nitinol wire is around $6. "Demonstration with a battery module
consisting of commercial 18650 lithium-ion cells shows that this thermal
regulator increases cold weather capacity by more than three-fold simply
by retaining the battery's self-generated heat...while also keeping the
module from overheating in hot environments even at high 2C discharge
rate," the researchers conclude.